The step back counter is highly effective in boxing and kickboxing, and of course mma. You just have to know how to use it. It's best against a rear arm straight punch from an opposite stance opponent.
So, say your southpaw, when you see an orthodox opponent throw a straight right, step back, landing on the ball of your rear foot, and immediately push off of that foot to shift your weight for the straight left counter over the top of their retracting arm. That example is actually the exact counter McGregor used against Aldo.
It's not as effective after a jab or a hook from the opposite side, because a combination 1-2 or 3-2 will often land on you before your counter will land. And it's not as effective against a same stance opponent's straight punch because the angle that you will be stepping back is straight back from their punch. However, it can be used against a front arm hook of a same stance opponent, because the angle is right. Korean Zombie's KO of Mark Hominick was a good example of this counter.
So to conclude, it's best to use against a punch coming from the same side as the arm you'll be countering with (eg. counter their right arm punch with your left, and vice versa), and don't use it against a jab.
If you want some examples of fighters who use this counter, I'd suggest watching Lyoto, McGregor, KZ, Saenchai (muay thai), and Pernell Whitaker (boxer)